Control means for defrosting a forced air unit



Nov. 17, 1970 A. cs. HERON 3,540,228

CONTROL MEANS FOR DEFROSTING A FORCED AIR UNIT Filed Aug. 20, 1968 United States Patent 3,540,228 CONTROL MEANS FOR DEFROSTING A FORCED AIR UNIT Andrew George Heron, Stocksfield, Northumberland, England, assignor to Heron Electrical Devices Limited Filed Aug. 20, 1968, Ser. No. 753,977 Int. Cl. F25b 15/00 U.S. Cl. 62140 5 Claims ABSTRACT OF THE DISCLOSURE A circuit for controlling the defrosting of a refrigerator unit incorporating a radiator with cooling fins, a refrigerator motor and a fan to blow air to be cooled through the radiator, comprising a first thermostat contact located near the radiator which contact is actuated by a rise in temperature to above freezing and restored by a drop in temperature to below freezing, a first reed contact, a flap moved by the draught from the fan, a magnet carried by said flap and actuating the reed contact, a first electromagnetic relay operated when both the thermostat and reed contacts are closed, and a break contact of said relay in series with the current supply to said refrigerator motor and fan.

The present invention relates to means for controlling the defrosting of a refrigerator unit, and has for its object the provision of apparatus for controlling the defrosting action in a cheap and reliable manner.

The invention is more particularly concerned with the use of a forced-air cooling unit for keeping a room or large container cool or cold. It is known that such forced aid cooling units are composed of a number of coils of piping to which radiating fins are fixed there being a fan for circulating air through the cooling unit into the cold room and back again, thus transferring a very large quantity of heat from the air to the cooler in a short time. It is also known that under these conditions the fins of the cooler rapidly pick up moisture, freeze it, and thus constrict the spaces between the fins through which the air is to be circulated. It is also known to use a defrosting mechanism which at specified intervals of time turns off the fan and the refrigerating pump unit, and applies current to an electric heater located within the cooling unit, this causing defrosting. After the action is complete, the refrigerating unit is put into operation again.

Whilst such a device is relatively simple, there may well be periods when, for example, since the cold room has not been opened, no defrosting action is necessary; if this defrosting action is then carried out, and unnecessary amount of heat is imparted to the cold room, representing lost power. Alternatively, too much frost may build up before defrosting commences, making continued use of the cooling unit ineflicient. The invention therefor has for its object the provision of apparatus for controlling a forced air unit and effecting defrosting in a simple and effective manner, at times determined only by the degree of frosting. The forced air unit is associated with refrigerating apparatus.

Reference should now be made to the accompanying drawings, in which:

FIG. 1 is a circuit showing the basic principles of the invention, and

FIG. 2 is an improvement on FIG. 1.

Both circuits are shown with their contacts in positions occupied during refrigeration.

Referring to FIGURE 1, it will be seen that there is a refrigerating pump motor M, a fan F, and a defrosting heater H. There is also a thermostat TH1 with a contact which opens above a temperature of F. and closes at 30 F., a second thermostat TH2 of which the contact opens above 36 F., and closes at 32 R, an air blast operated contact P which remains open so long as there is sufiicient air flowing through the fins of the cooler, and a relay R having contacts R1 and R2. The contact P is a reed switch, operated by a magnet carried on a hinged flap X located in the path of the draught from the fan after it has passed through the cooling fins. So long as the draught is adequate, the flap is held away from the switch, but as frost accumulates the draught decreases so that the flap falls slowly, finally actuating the switch. The flap has a large hole in its centre with an adjustable shutter for the hole, the amount of draught to operate the switch being determined by the amount of occlusion of the hole by the shutter. The operation of the circuit is as follows:

Current is supplied to the apparatus from the line wires L and N. The circuit is shown in the normal operating condition i.e. one in which the refrigerator motor M is running together with the fan F. Since the ambient temperature in the region of the thermostats TH1 and TH2 both of which are mounted near the fins of the cooler, is below 30 F., it follows that'both their contacts are closed. The motor and fan thus run normally, the draught through the fins of the cooler maintaining the flap operating the contact P in a swung-away position so that its magnet leaves the contact P open.

When frost builds up on the fins of the cooler to such a degree that the draught through the cooler is reduced below the predetermined limit, the flap X operating the contact P falls gradually, and the contact P finally closes. This sets off a delayed action contact D, which after a period of say 20 seconds to allow for conditions to settle down, also closes. This operates relay R through contacts TH1 and P to line and neutral.

Contact R1 closes and supplies current to the defrosting heater H, and contact R2 opens the circuits of the refrigerator motor M and the fan F. This condition persists until the temperature rises above 36 F., when contact TH2 opens. This does nothing since contact R2 is open. When the temperature rises above about 50 F., contact TH1 opens and though contacts P and D are both closed, relay R releases once more. This turns off the heater H and starts up the motor M. The radiator now cools until at 32 F., TH2 resets and starts up the fan F. The fan blows the flap X away from the vicinity of the contact P, which also opens, so that relay R remains unoperated and the delay contact D opens, the circuit acting to cool the air through the fins until the temperature falls below 30 F., when TH1 resets. The circuit is now in its normal operative state.

When the fin temperature rises above freezing, the frost thereon melts and drips away into a trough. The delay in re-starting the fan after defrosting ensures that if any drops of water are left'on the fins, they are frozen in place before the fan starts so that they are not blown out into the cooled chamber.

FIG. 2 shows a circuit of a fail-safe device which prevents reapplication of current to the heater if the fan fails to start up properly after defrosting. In this circuit contact TH1 is a thermostat which changes over above 50 F. and changes back below 30 F., and contacts PA and PB are reed switches operated by the same flap X. Both contacts close when the draught through the radiator fins fails either from frosting or from non-operation of the fan, and both contacts open when the fan is switched on after de-frosting.

The circuit is shown in a normal condition, in which current from the line Wires L and N is supplied via contact RA2 to the motor M and fan F. The motor operates the refrigerating plant and the fan blows air through the cooling fins into the chamber. Thermostat TH1 remains in the position shown so long as the temperature of the room is low. The contact PA remains open so long as an adequate draught is received by the associated flap X through the fins.

When the fins become so frosted that the air blast drops to an insufficient value, the flap X associated with the two contacts PA and PB falls, and the contacts close. PA operates the delay contact D after a period of 20 seconds, and current from line L traverses contact TH1, PA, D and RB2 to operate relay RA, which has contacts RA1 and RA2.

Contact RA1 turns on the heater H and contact RA2 disconnects the motor and fan F both of Which stop. The heater defrosts the cooling fins, and when the temperature of the fins rises above 50 F., contact TH1 changes over, releases relay RA and operates relay RB, which has contacts RBI and RB2. RBI locks relay RB operated via contacts PB which is closed since the fan is not running. Contact RBZ prevents re-operation of relay RA, which relay was de-energised when contact TH1 changed over. Contact RA1 disconnects the heater and contact RA2 starts up the motor and fan F.

If the fan motor starts satisfactorily and provides adequate blast, contacts PA and PB open, and later when the temperature of the fins has dropped sufliciently, contact TH1 changes back again to the position shown. This releases relay RE, the contacts of which do nothing at this particular moment.

If, however, the fan fails to start up, then contact PB remains closed and irrespective of the position of the thermostat TH1, relay RB remains held operated through contact PB and contact RBI. Contact RB2 thus remains open, and even if the thermostat TH1 returns to its previous condition, relay RA cannot be re-operated, thus ensuring that the heater is not switched on once more. Thus the circuit fails safe in that until the temperature of the finsh as been adequately lowered and the fan has become operative again, relay RA cannot reoperate. This condition is highly desirable in that whilst no harm is done to the contents of the cool chamber if a certain degree of excessive frosting takes place, repeated heating of the radiator fins by the heater H could easily spoil the contents of the chamber.

Although no thermostat corresponding to TH2 of FIG. 1 is shown in the present FIG. 2, this thermostat could obviously be added if it were found desirable.

I claim:

1. A control system for defrosting a refrigerator unit having a radiator with cooling fins, a refrigerator motor and a fan to blow air to be cooled through the radiator, comprising a first thermostat switching means that is deenergized by a rise in temperature beyond a predetermined point above the freezing point and energized by a drop in temperature beyond a predetermined point below the freezing point, a reed switch, a flap moved by the draught from the fan, a magnet carried by the flap and actuating the reed switch, an electromagnetic relay energized when both the thermostat switching means and the reed switch are energized, a relay switch means connected in series with the refrigerator motor and fan, and a second thermostat switching means connected in series with the fan, the second thermostat switching means being de-energized at a predetermined temperature above the freezing point and being energized at the freezing point.

2. A control system as recited in claim 1, further including a delay switch connected in series with the first electromagnetic relay.

3. A control system for controlling the defrosting of a refrigerator unit incorporating a radiator with cooling fins, a refrigerator motor and a fan for blowing air to be cooled over the cooling fins, comprising in combination a first thermostat switching means located near the radiator and energized to a closed position in response to a fall in temperature to below a preset temperature value below freezing and de-energized to an open position in response to a rise in temperature to above a preset maximum temperature value above freezing, a first reed switch in series with said first thermostat switching means, a flap moved by the draught from the fan, a magnet carried by said flap for closing said reed switch in response to a reduction of draught below a predetermined minimum value, a first electromagnetic relay connected in series with said first thermostat switching means and said first reed switch and operative in response to either closing of said first reed switch or opening of said first thermostat switching means, a first relay switch connected in series with said refrigerator motor and said fan and opened by operation of said first relay in response to closing of said first reed switch and closed by operation of said first relay in response to opening-of said first thermostat switching means, a resistor heater, and a second relay switch connected in series with said resistor heater and closed by operation of said first relay in response to closing of said first reed switch and opened by operation of said first relay in response to opening of said first thermostat switching means.

4. A control system according to claim 3, wherein said refrigerator motor and said fan are connected in parallel and a second thermostat switching means is located near said radiator and connected in series with said fan, said second thermostat switching means being opened in response to a rise in temperature above a preset temperature value above freezing but below the preset maximum temperature value of said first thermostat switching means, and being closed in response to a fall in temperature to below freezing.

5. A control system according to claim 3, including a second electromagnetic relay wherein said first thermostat switching means is a thermostatic change-over switch whereby movement of said first thermostat switching means to its open position operates said second electromagnetic relay, the control means further comprising a second reed switch connected in parallel with the current supply when said first thermostat switching means is in an open position and being closed by the said magnet in re sponse to a reduction of draught from the fan below the said predetermined minimum value, a first relay switch of said second relay connected in series with said second reed switch and acting when closed to lock said second relay in its operative condition, and a second relay switch of said second relay connected in series with said first relay and acting when open to prevent re-operation of said first relay until the fan has positively restarted.

References Cited UNITED STATES PATENTS 2,962,870 12/1960 Von Arb 62140 2,975,611 3/1961 Pietsch 62-140 3,077,747 2/1963 Johnson 62140 MEYER PERLIN, Primary Examiner US. Cl. X.R. 

